StartActivityForResult with MVVM Architecture Components

Question

In one of the Google Codelabs, Activity1 displays a RecyclerView and on button click opens Activity2 where a new entry can be added.

Instead of accessing a ViewModel in Activity2 and adding the entry to the database directly, they send the entry back to Activity1 and insert it there.

What is the reasoning behind this? Why send the data back to Activity1? Why not call ViewModelProviders.of(this) in Activity2 and insert the entry in there?

FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent = new Intent(MainActivity.this, NewWordActivity.class);
            startActivityForResult(intent, NEW_WORD_ACTIVITY_REQUEST_CODE);
        }
    });
}

public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == NEW_WORD_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {
        Word word = new Word(data.getStringExtra(NewWordActivity.EXTRA_REPLY));
        mWordViewModel.insert(word);
    } else {
        Toast.makeText(
                getApplicationContext(),
                R.string.empty_not_saved,
                Toast.LENGTH_LONG).show();
    }
}

Source: https://github.com/googlecodelabs/android-room-with-a-view/blob/master/app/src/main/java/com/example/android/roomwordssample/MainActivity.java

post how u sending response from NewWordActivity back like set result or something! — Shishram, Sep 29 '18 at 10:34
Because, Google suggests that avoid using Single `ViewModel` due to lifecycle pitfalls to activity, that's why they avoid to use same instance `ViewModel` in second activity & pass data to first activity instead. — Jeel Vankhede, Sep 29 '18 at 10:39
But the second activity could just use the `ViewModelProviders.of` method to get it's own ViewModel instance` — Florian Walther, Sep 29 '18 at 10:41
IMHO, Google has no monopoly on the truth. They make bogus decisions too. — beroal, Dec 25 '18 at 16:48
Because they don't want to create two instances of WordViewModel for two activities. If they use two fragments then yes we can do like what you suggest above, getting the viewModel instance and saving your data. — Khoa Tran, Apr 24 '19 at 03:35

score 0 · Answer 1 · answered Sep 29 '18 at 10:37

0

Looking at the NewWordActivity class it's very simple and doesn't require its own ViewModel. My guess is that it's to avoid the need to create one for the single purpose of inserting an entry into the database, giving that responsibility to the MainActivity.

answered Sep 29 '18 at 10:37

Ivan Wooll

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1

Thank you. Would you still use `startActivityForResult` when there also should be an update feature? – Florian Walther Sep 29 '18 at 10:39
It wouldn't be necessary if you are using LiveData to observe the database in the first Activity. – Ivan Wooll Sep 29 '18 at 12:18
1

With update I meant updating the entry in the database – Florian Walther Sep 29 '18 at 12:20
The principle is the same whether you are inserting, updating or deleting entries into your database. Observing with LiveData means that your activity doesn't need to know when to go and check for changes as any changes are automatically propagated to Observers. – Ivan Wooll Sep 29 '18 at 12:25
1

Yes but I am trying to understand if I should still use startActivityForResult then – Florian Walther Sep 29 '18 at 12:27
What do you expect to be doing when you get a result from an Activity? – Ivan Wooll Sep 29 '18 at 12:29
1

The example in my question sends the data to activity1 and inserts it there – Florian Walther Sep 29 '18 at 12:30
1

I mean, that's what this question was about, right? – Florian Walther Sep 29 '18 at 19:51
1

Simply calling `ViewModelProviders.of` and `insert` in the 2nd activity would be less work than handling intent extras and activity result, right? So what is the reasoning behind sending the data to the `MainActivity`? – Florian Walther Oct 01 '18 at 09:40
@FlorianWalther this was exactly my question when I watched your youtube MVVM tutorial. LOL . thanks for the tutorial btw – sarah Mar 23 '20 at 23:30

StartActivityForResult with MVVM Architecture Components

1 Answers1